Third Release Candidate for GIMP 2.4

GIMP 2.4.0-rc3 fixes most of the bugs that were reported for the earlier release candidates.

Changes in GIMP 2.4.0-rc3
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- use the new format for storing recently used files
- added conversion options to the color profile conversion plug-in
- allow to disable the toolbox menu on all platforms
- further improved handling of the JPEG settings
- switch tabs when hovering over them
- added a PDB function to remove the alpha channel from a layer
- plug-in previews remember the state of the Preview checkbox (bug #478657)
- allow to grow the image/layer using the Crop tool
- bug fixes